Ultrasound is commonly used for imaging needle insertion during interventional surgical procedures. Such needles may, for example, be paracentetic needles for withdrawal of fluid from a target anatomy in a body or biopsy needles for the removal of samples from a target anatomy in a body. Needles may also be used to administer drugs or other substances into a specific location relative to a target anatomy in the body.
During such procedures the precision of needle trajectory is highly important. The needle should not disturb or damage anatomy not connected with the procedure being undertaken. Some procedures may be directed towards small regions in the body e.g. artery or an area close to a major anatomy, such as the heart. Accuracy of needle placement is therefore vital.
Ultrasound is often used as an imaging method for imaging a needle and a target anatomy in support of the interventional procedure. Other imaging alternatives, such as Xray or EM (electromagnetic) guidance, suffer from drawbacks such as risk of radiation exposure, 2D view only, additional set-up time, tracking errors or lack of visibility of the specular object, such as a biopsy needle.
A method according to the preamble for imaging a specular needle together with a target anatomy in a tissue is described in “Enhancement of needle visibility in ultrasound-guided percutaneous procedures”; Cheung and Rohling's; Ultr. Med. Biol. (2004) 30: 617-624. Here the visualization of a needle in the ultrasound image is enhanced, without compromising the quality of the visualization of the target anatomy in the tissue, by displaying the combination of a specular object image (also referred to as a needle-specific image) and a tissue image. The tissue image, obtained using a tissue mode specific set of parameters, depicts the target anatomy in the tissue but may depict the needle in a poor quality, or even do not depict the needle at all, whereas the specular object image, obtained using a specular object mode specific set of parameters, depicts the needle with great contrast, but may depict almost no background tissue. Combining both images, by taking their weighted average, yield an image (often referred to as a needle-enhanced image) showing both the needle and the target anatomy.
The characteristics of the ultrasound imaging in the tissue imaging step and in the specular object imaging step are optimized to allow the best possible responses. These characteristics are set by using respectively the tissue mode specific set of parameters or the specular object mode specific set of parameters. These sets of parameters may comprise, for example, the frequency of the ultrasound waves, beam steering and beam density related parameters, the focal depth, the rate at which the echoes are received (the frame rate), and parameters controlling the type of processing of the received echoes, such as for example aperture processing and temporal filtering.
In the above described method the favored geometry for the needle to be displayed is the so-called “in-plane” geometry wherein the long axis of the needle is contained in the target plane including the target anatomy. However, in a significant number of cases the needle location deviates from the target plane due to, for example, tissue heterogeneities and bevel asymmetry.
It is now a problem of the known method that such an out-of-plane needle is not shown (or not sufficiently shown) in the combined (needle-enhanced) images. The clinician has to move the imaging transducer, transmitting and receiving the sound waves, to locate the needle, but then loses the target anatomy located in the original target plane. Furthermore, since the clinician does not know where the needle is located in relation to the original target plane, he has no indication how to move the transducer to find the needle.